Pathological anatomies such as tumors and lesions can be treated with an invasive procedure, such as surgery, which can be harmful and full of risks for the patient. A non-invasive method to treat a pathological anatomy (e.g., tumor, lesion, vascular malformation, nerve disorder, etc.) is external beam radiation therapy. In one type of external beam radiation therapy, an external radiation source is used to direct a sequence of x-ray beams at a tumor site from multiple angles, with the patient positioned so the tumor is at the center of rotation (isocenter) of the beam. As the angle of the radiation source changes, every beam passes through the tumor site, but passes through a different area of healthy tissue on its way to the tumor. As a result, the cumulative radiation dose at the tumor is high and the average radiation dose to healthy tissue is low.
The term “radiotherapy” refers to a procedure in which radiation is applied to target regions for therapeutic, rather than necrotic, purposes. The amount of radiation utilized in radiotherapy sessions is typically about an order of magnitude smaller, as compared to the amount used in a radiosurgery session. Radiotherapy is typically characterized by a low dose per treatment (e.g., 100-200 centiGray (cGy)), short treatment times (e.g., 10 to 30 minutes per treatment), and hyperfractionation (e.g., 30 to 45 days of treatment). For convenience, the term “radiation treatment” is used herein to include radiosurgery and/or radiotherapy, unless otherwise noted.
Traditionally, medical imaging was used to represent two-dimensional views of a patient. Modern anatomical imaging modalities such as computed tomography (CT) are able to provide an accurate three-dimensional model of a volume of a patient (e.g., skull or pathological anatomy bearing portion of the body) generated from a collection of CT slices. Each CT slice corresponds to a cross-section of the patient. These CT slices are typically obtained every 1.25 or 3 millimeters so that a set of images represents a three-dimensional model of the volume of interest.
Conventional treatment planning software packages are designed to import 3D images from a diagnostic imaging source such as magnetic resonance imaging (MRI), positron emission tomography (PET) scans, angiograms, and computerized x-ray tomography (CT) scans. During treatment planning, volumes of interest (VOI) from anatomical (e.g., CT) and/or functional imaging are used to delineate structures to be targeted or avoided with respect to the administered radiation dose. FIG. 1 illustrates a conventional contour set which may be used to define a volume of interest (VOI) structure. The contour set includes multiple image slices, including end slices and a middle slice. The volume of interest structure may be defined as a set of planar, closed polygons, within a plurality of image slices. The coordinates of the polygon vertices are defined as the x, y, and z offsets in a given unit from an image origin. Due to limited processing power, conventional treatment planning systems typically do not use every two-dimensional slice within a set. Rather, conventional treatment planning systems use linear interpolation between non-adjacent slices (e.g., every tenth slice) to minimize the time and power allocated to defining the volume of interest structure. However, linear interpolation fails to account for pathological anatomy formations such as indentations and protrusions that are only visible on the middle slices ignored and replaced by the interpolated contours.
Volume of interest structures may include target regions and critical regions. A target region is a volume of interest structure to which radiation is directed for therapeutic or surgical purposes. A critical region is a volume of interest structure for which radiation treatment is avoided. For example, a CT slice of a spinal region may include a pathological anatomy (e.g., tumor, legion, arteriovenous malformation, etc.) target region to be treated and an adjacent normal anatomy (e.g., internal organ) critical region to be avoided. The treatment planning software enables delineation of the target and critical regions on the two-dimensional CT image slices. Conventionally, a user manually delineates points on the two-dimensional image represented on a medical imaging display to generate a corresponding contour. Ideally, the volume of interest contours for all of the slices should match the corresponding target or critical region over its three-dimensional volume. Such matching is difficult due the three-dimensional nature and irregularities of the pathological and normal anatomies. For example, two-dimensional delineation is of limited applicability for complex volume of interest structures such as vascular structures.